Add these lines in [your_game]\Config\DefaultEngine.ini and change the value of MaxLODSize=4096 to MaxLODSize=8192 for the TextureGroup you want.
[SystemSettings]
TEXTUREGROUP_World=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_WorldNormalMap=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_WorldSpecular=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_Character=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_CharacterNormalMap=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_CharacterSpecular=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_Weapon=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_WeaponNormalMap=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_WeaponSpecular=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_Vehicle=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_VehicleNormalMap=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_VehicleSpecular=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_Cinematic=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_Effects=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=linear,MipFilter=point)
TEXTUREGROUP_EffectsNotFiltered=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_Skybox=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_UI=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_Lightmap=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_Shadowmap=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point,NumStreamedMips=3)
TEXTUREGROUP_RenderTarget=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_MobileFlattened=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_Terrain_Heightmap=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_Terrain_Weightmap=(MinLODSize=1,MaxLODSize=4096,LODBias=0,MinMagFilter=aniso,MipFilter=point)
TEXTUREGROUP_Bokeh=(MinLODSize=1,MaxLODSize=256,LODBias=0,MinMagFilter=linear,MipFilter=linear)